This invention serves to solve an engine start problem in an annular type of combustor that includes a plurality of equally spaced dual fuel nozzles feeding fuel into the combustor. In the context of this patent application the term dual nozzle is a single fuel nozzle that houses both a primary and secondary flow system and each system is referred to as the primary nozzle and secondary nozzle, respectively. In this type of burner construction, as for example, that exemplified by the JT9D engine model manufactured by Pratt & Whitney Aircraft Group of United Technologies Corporation, the assignee of this patent application, each fuel nozzle is identical and each fuel nozzle delivers substantially the same amount of fuel to the combustor. Typically, in such a dual nozzle the primary nozzle is operable over the entire engine operating envelope and delivers, say 75% of fuel during idle and 10% during takeoff. The secondary nozzle, typically is utilized solely during higher thrust operations and delivers 30% of the total fuel during idle and 90% during takeoff. The total fuel flow is metered by a fuel control and distributed by a diverter valve to the fuel manifolds communicating with these nozzles.
In a conventional gas turbine engine, as the JT9D engine model, supra, starting fuel flow (600-800 pounds per hour total) is provided to all fuel nozzles, and as noted above, the distribution is even through all the nozzles. The fuel is typically atomized by pressure or aerated and mixes with air to form a combustible mixture. Combustion is initiated by exposing this mixture to a high energy igniter which remains activated for a given period of time, until at least after, combustion ensues.
Because of mechanical limitation within the combustor and engine, such as starting during other than optimum starting temperatures (as for example cold days) or where the nozzle and igniter design configuration may not be at optimum, the operator may induce increased amounts of fuel flow to achieve starting. Under such circumstances, the starting times tend to become longer and fuel tends to accumulate or form puddles in the combustion chamber, consequently causing undesirable hot starts with a tendency of increasing the incidence of compressor stall.
We have found we can improve the incidence of starting and starting times by making an otherwise symmetrical nozzle/fuel injection system in an annular combustor into an asymmetrical system. It is contemplated by this invention that simple flow restrictors are incorporated in all of the non-igniter located primary nozzles and the igniter located primary nozzles remain in tact (unrestrictive). Hence, in an already existing system, the starting characteristics are improved by merely adding flow restrictors at judicious locations--say in a twin igniter system, 18 of 20 nozzles are restricted. This allows the use of existing hardware and does not necessitate the change of the fuel flow requirements and its fuel metering and distribution systems.
Other means for achieving optimum starting are contemplated within the scope of this invention, as for example, plugging a limited number of primary nozzles, changing the flow characteristics of the igniter located nozzles, and the like. The important aspect of this invention is that the amount of fuel distributed to the igniter to obtain optimum starting characteristics is preascertained and the fuel from the distribution system distributing an already established fuel flow is redistributed to an asymmetrical distribution pattern to achieve the requisite predetermined amount of the fuel to the igniter nozzle.
This serves to limit the quantity of fuel in the restrictive nozzles while increasing the quantity in the unrestrictive nozzles. The amount of fuel selected in the unrestrictive nozzles is preascertained to achieve optimum starting. Hence the size of restrictor is predetermined to achieve the quantity of flow necessary in the unrestrictive nozzle.